Method of avoiding excessive polarization mode dispersion in an optical communications link

ABSTRACT

A method and apparatus for managing state-of-polarization (SOP) controllers along an optical communications link. When the polarization-mode dispersion (PMD) in the link exceeds a preset limit as determined by a PMD compensator, the SOP controller is directed to change rotation angle in an arbitrary fashion. This shift is only undertaken after other indicators are examined to determine that PMD is the main problem affecting the link, and not simply a consequence of a separate problem that the SOP controller rotation angle should not try to fix. By reducing unnecessary SOP adjustments which can disrupt data traffic, the reliability of the optical link is improved.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] Cross reference is made to the following co-pending patentapplications, each being assigned to the same assignee as the presentinvention and the teachings included herein by reference: SER. NO. TITLEFILING DATE ATTORNEY'S DOCKET FAULT ISOLATION OF AN HEREWITH RIC-98-012OPTICAL LINK BY CORRELATING PMD EVENTS WITH OTHER MEASUREMENTS

FIELD OF THE INVENTION

[0002] The present invention is generally related to opticalcommunications systems, and more particularly to a method of limitingpolarization-mode dispersion (PMD) in the optical link includingreducing unnecessary corrective actions based upon a limit conditiondeclared by a PMD compensator.

BACKGROUND OF THE INVENTION

[0003] In a typical optical communications system, an optical signal isemitted from a modulated laser diode comprising an optical transmitterin the form of a series of light pulses. Each light pulse is ofextremely short duration, such as 40 ps, and is roughly Gaussian shapedas a function of time. In the frequency domain, this signal comprisesnumerous frequency components spaced very closely about the nominalcenter frequency of the optical carrier such as 193,000 GHz. As thistype of modulated optical signal passes through an optical fiber,different frequency components of the optical signal travel at slightlydifferent speeds due to an effect known as chromatic dispersion. In thecourse of an optical signal traveling through a very long fiber, such as200 km, chromatic dispersion causes a single pulse of light to broadenin the time domain, and causes adjacent pulses to overlap one another,interfering with accurate reception. Fortunately, many techniques areknown for compensating for chromatic dispersion.

[0004] Another form of dispersion is becoming a limiting factor inoptical communications systems as progressively higher data rates areattempted. Polarization-mode dispersion (PMD) arises due tobirefringence in the optical fiber. This means that for two orthogonaldirections of polarization, a given fiber can exhibit differingpropagation speeds. A light pulse traveling through a fiber willprobably, unless some control means are employed, have its energypartitioned into polarization components that travel at differentspeeds. As with chromatic dispersion, this speed difference causes pulsebroadening and restricts the usable bandwidth of each optical carrier.

[0005] A modulated optical signal arriving at an optical receiver mustbe of sufficient quality to allow the receiver to clearly distinguishthe on-and-off pattern of light pulses sent by the transmitter.Conventionally, a properly designed optical link can maintain abit-error-rate (BER) of 10⁻¹³ or better. Noise, attenuation, anddispersion are a few of the impairments that can render an opticalsignal marginal or unusable at the receiver. Generally, when an opticalchannel degrades to a bit-error-rate of 10⁻⁸, a communications systemwill automatically switch to an alternate optical channel in an attemptto improve the BER.

[0006] One common method of analyzing the quality of a modulated opticalsignal is a so-called “eye diagram”, shown in FIG. 1. The eye diagramconsists of overlaying successive frames of time-domain traces of thesignal, with each frame corresponding to one period of the nominalperiodicity of the modulation. As portrayed, the vertical axisrepresents instantaneous intensity of the received signal, and thehorizontal axis corresponds to time. Many successive traces oftransmitted “ones” and “zeros” define a region or window within themiddle of the display. In the time axis, the window is bound on eitherside by the transitional leading and trailing edges of the pulses. Usingthis technique, a large clear area or “window” in the center with noencroachment from any side represents a good signal in that the presenceof absence of a pulse during each clock cycle is clearlydistinguishable.

[0007] Noise added to a signal appears as “fuzziness” of the linesdefining the window. Sufficient noise can even obliterate the appearanceof the window, representing a bad signal in that “ones” and “zeros” areno longer distinguishable. Impairments in the time axis, such aschromatic dispersion or polarization mode dispersion, cause thetransitional areas of the display to close in upon the window fromeither side. Overlapping of pulses can require more stringentsynchronization of the receiver's decision point, or even render thesignal unusable.

[0008] A given optical receiver will automatically adapt to receive amodulated optical signal. Automatic gain control (AGC), frequencycontrol, and phase lock-in are typically applied in sequence so that athreshold decision circuit can best sample the signal and decode everypulse. Superimposed upon the eye-diagram, an optimal point of operationfor a threshold decision circuit would intuitively be at the center ofthe window, as shown by the “+” in FIG. 1. This means that the intensitythreshold is about halfway between the zero values and one valuesobserved on average.

[0009] Timewise, the center of the window corresponds to sampling thepulses in the middle of their duration when they tend to be of maximumintensity also shown by the “+” in FIG. 1. Intuitively, one can see howthis choice for an operating point would be the most robust againsteither noise or timing impairments which cause the window to shrink.

[0010] A received optical signal can undergo some degree of change in,for example, pulse width without having an immediate impact on BER asobserved by this optimally positioned main decision circuit. Aparticular type of receiver has been developed comprising at least twoindependent decision circuits of the type just described. A maindecision circuit is dedicated to actual communications reception and ismaintained at the optimum point, once it is established, within thecenter of the window. But for analyzing signal quality to a finer degreeand for measuring degradation before it impacts BER of the actualcommunications, an auxiliary decision circuit is used to probe theextents of the operating window. Robustness to timewise disturbances isgaged by directing the auxiliary decision circuit to sample at varioustime offsets relative to the optimum point. Findings by the auxiliarycircuit may even be used to fine-tune the optimum point settings of themain decision circuit.

[0011] The auxiliary decision circuit is set to a given timing offsetand its output is monitored for BER, especially in comparison to theoutput of the main decision circuit. The BER measurement at eachoperating point can typically take several minutes. Gradually, BER datais accumulated for every offset value. As expected, a plot of this dataresembles an inverted Gaussian curve with a minimum BER occurring someoptimum offset, as shown in FIG. 2. A similar plot is derived by varyingthe amplitude threshold of the auxiliary decision circuit.

[0012] All of this BER data may be summarized into a “Q” factor orquality metric for the received signal. In general terms, the broaderthe range of timings over which a low BER can be sustained, the greaterthe Q factor of the signal. A receiver with an auxiliary decisioncircuit can measure and output such a Q factor.

[0013] During the time that an auxiliary decision circuit isaccumulating measurements to compile a Q factor for a received signal, ashift in dispersion characteristics, particularly PMD characteristics,can take place along the fiber. This can result in an inaccurateassessment of the signal quality, especially if a PMD controller (PMDC)cannot quickly and sufficiently compensate for the PMD change.Therefore, the Q factor cannot be solely relied upon as a measure ofpath quality.

[0014] The Q measurement is particularly useful for assessing andfine-tuning an optical path that is already operating at a healthy lowBER. The Q measurement estimates a BER without requiring any actual biterrors to occur. A Q measurement covering the BER range of 10⁻¹³ to10⁻²⁰ may be completed in a few minutes, whereas an actual errored bitmight not be observed for 5 hours, days or months.

[0015] Schemes to actively compensate for PMD generally involvedetecting the presence of polarization-dependent timing differences andeither a) applying delay elements to one or the other polarization torealign the timing of pulses or b) controlling the polarization state ofthe signal upon entry into the fiber, or at intermediate points alongthe fiber, such that birefringent effects are minimized or canceled out.Active compensation techniques are required because the PMD of a givenfiber varies over time due to temperature and pressure changes along thefiber, and due to aging. A fiber that is installed above ground canexhibit fairly rapid fluctuations in PMD due to temperature andmechanical forces. A fiber buried underground can be sensitive to loadssuch as street traffic or construction work.

[0016] PMD of the optical carrier along the optical fiber may be reducedby the placement of one or more state-of-polarization (SOP) controllersalong the fiber. A SOP controller can output light at one of severalselectable polarization states. For optical communications, the relativerotation angle is the main property that is controlled. By adjusting thestate-of-polarization of an optical carrier as it enters the fiber, thePMD effects of the fiber can be minimized. Furthermore, SOP controllersapplied at several points along a fiber can effectively use the fiber'sown characteristics to cancel PMD.

[0017] A common PMD controlled optical path comprises a transmitter, afiber with one or several SOP controllers, a PMD controller (PMDC) and areceiver. For a modulated optical signal, the PMDC can sense the timingdifference between a pair of orthogonal polarizations and selectivelydelay one polarization to realign the timing between the two signalhalves before passing the signal to the receiver. The receiver expects asignal with less than a certain amount of chromatic andpolarization-mode dispersion. As the polarization characteristics of thefiber change, the PMDC constantly monitors the signal and adjusts tominimize the PMD contribution to overall dispersion.

[0018] A problem arises in that the PMDC typically has a finite range ofdelay compensation that it can accomplish. Normally, the SOPs along afiber are set such that, on average, the bulk of the PMD is curtailedbefore the PMDC. However, on some occasions, the PMD of a given fibercan drift beyond the range of the PMDC.

[0019] When the PMD of the optical path degrades, some corrective actionmay be necessary either to improve the PMD of the optical path or todivert the communications traffic along an alternate channel or paththat will work better. Yet, it is equally important to the integrity ofthe traffic bearing signal to avoid taking unnecessary correctiveactions. Each adjustment or switching operation can temporarily disruptthe revenue bearing traffic signal.

[0020] There is desired a way to quickly restore the integrity of anoptical link under these conditions, including improving PMD, whilereducing corrective actions based upon limit conditions declared by aPMDC.

SUMMARY OF THE INVENTION

[0021] The present invention achieves technical advantages as an opticalcommunication system having a SOP controller along an optical link thatis intelligently and responsively directed to change rotation stateswhenever a PMDC surpasses a preset compensation limit. The SOPcontroller is initially set such that the likelihood of the PMDCreaching a compensation limit is acceptably small. Upon the rareoccasion that the compensation limit is exceeded, one or more of the SOPcontrollers are commanded to alter rotation of polarization by somearbitrary amount, for example, 45 degrees to 90 degrees. This rotationaction will most likely result in lowering the end-to-end PMD toacceptable levels. The rare condition of high PMD occurs when the fibershifts properties and exhibits a worst case dispersion in conjunctionwith the settings of the SOP controllers. Under worst conditions,changing any of the SOP controllers in either direction will most likelyresult in reduced PMD.

[0022] According to the present invention, since this rotation actioncan cause a momentary disruption in the traffic-bearing optical signal,abrupt changes in SOP controllers are intelligently made along anoptical link only when absolutely necessary to preserve the integrity ofthe link. The present invention utilizes intelligent fault detectionprocessing of the optical system to provide a safeguard against falsetriggering and making unnecessary adjustments to a SOP controller. Acontroller intelligently processes several indicators from systemdevices, including notifications from the PMDCs, to determine whetherthe limit condition declared by a PMDC is the main problem affecting thelink, or if the limit condition is simply a consequence of a separateproblem that the PMDC should not try to fix.

[0023] The present invention correlates several indicators tointelligently generate the switch rotation state indication for the SOPcontroller, including notifications from PMDCs, a detected BER, Q factorand signal-to-noise ratio (SNR), to decide whether a path degradation istruly attributable to PMD or other causes. The present inventionprovides a technique to gate whether or not the SOP controller shouldabruptly shift states.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 illustrates an eye diagram which is typically used toanalyze the quality of a modulated optical signal;

[0025]FIG. 2 is a graph of a typical BER measurement as a function ofoffset which can be used to determine a Q factor of an optical signal;

[0026]FIG. 3 is a block diagram of an optical communications systemaccording to the present invention having a controller adapted toperform the method of the present invention;

[0027]FIG. 4 is a flow chart illustrating how the controller ascertainsand processes several inputs from the optical communication network todistinguish between fiber failure, PMD-related degradations, and failureof monitoring equipment according to the preferred embodiment of thepresent invention;

[0028]FIG. 5 is a flow chart illustrating how a Q measurement iscalculated and recorded as a valid recent Q measurement for use in thealgorithm of FIG. 4; and

[0029]FIG. 6 is a flow diagram of the algorithm of the present inventionfor intelligently changing the rotation angle of the PMD compensator asa function of PMDC alarms.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0030] Referring now to FIG. 3, there is illustrated a block diagram ofan optical communication system 20 according to the present invention.System 20 includes a microprocessor based controller 22 receiving andprocessing several indicator inputs from the system to perform faultmanagement according to the preferred embodiment of the presentinvention. System 20 is seen to include an optical transmitter 24including a semiconductor laser emitting light that is intensitymodulated by a corresponding electrical data signal provided on inputline 26. The electrical data signal can be a SONET-compliant STS-48 orSTS-192 synchronous data signal bearing digital data at about 2.5 Gbpsor 9.9 Gbps, respectively. The intensity modulated optical carrier isprovided by transmitter 24 into optical fiber 28. The optical carriermay be a SONET OC-48 or OC-192 signal bearing digital data atapproximately 2.5 or 9.9 Gbps, respectively. The optical fiber 28 mayinclude an optical amplifier generally shown at 30 for ampliflying theoptical carrier along the length thereof. It is noted transmitter 24 mayinclude several semiconductor lasers, each providing light that isintensity modulated by a corresponding input electrical data signal toprovide Wavelength Division Multiplexing (WDM) if desired. For purposesof teaching and illustrating the present invention, a singlesemiconductor laser generating a single optical carrier is discussed toteach and describe the present invention with it being understood thatthe present invention can apply to each of several or all opticalcarriers and being communicated over a common optical fiber.

[0031] Still referring to FIG. 3, optical system 20 can be seen toinclude a state of polarization (SOP) controller 31 and a polarizationmode dispersion controller (PMDC) 32 provided along optical fiber 28.The SOP controller 31 and the PMDC 32 both compensate thepolarization-dependent timing differences of the optical signal. The SOPcontroller 31 sets a fixed rotation angle of the optical signal tocurtail PMD before the PMDC 32. For a modulated optical signal, the PMDC32 actively senses the timing difference between the pair of orthogonalpolarizations and selectively delays one polarization to realign thetiming between the two signal halves before passing the signal to areceiver. As the polarization characteristics of the fiber change, thePMDC 32 constantly monitors the optical signal and adjusts the delay tominimize the PMD contribution to overall dispersion. PMDC 32 alsoprovides several PMD notifications to controller 22 on output line 34,as will be discussed further shortly.

[0032] In practice, one or several PMDCs 32 can be provided along thelength of optical fiber 28, and can be provided at several locationsincluding proximate the transmitter 24 for providing forwardcompensation, in the middle of the link, proximate the receiver, or anycombination thereof depending on the design of the optical communicationsystem. Thus, limitation to the location or number of PMDC 32 is not tobe inferred in the present invention.

[0033] Optical system 20 is further seen to include a signal-to-noiseratio (SNR) meter 36 receiving a tapped a portion of the optical carrierfrom an optical tap 37 coupled to the fiber link 28. SNR meter 36provides an indicator indicative of the signal-to-noise ratio of theoptical carrier on output line 38.

[0034] The optical fiber 28 terminates at the receive end at an opticalreceiver 40. The optical carrier transmitted through fiber 28 isdetected by a corresponding photo detector 44 generating a signal in theelectrical domain. The output of the optical photo detector 44 isprovided on output line 46 and provided to an electrical splitter 48.Splitter 48 divides the electrical data signal, providing one part to aprimary decision circuit 52 and the other second part to a secondarydecision circuit 54.

[0035] The primary decision circuit 52 includes circuitry determiningwhether or not the received electrical signal in the time domain is alogic one or a logic zero. This primary decision circuit 52 is set to anoptical setting, and has adjustable settings to adjust the decisionpoint as a function of the circuit design and information from theauxiliary decision circuitry to maximize the bit error rate (BER). Theprimary decision circuit 52 handles the usable data traffic and providesthe output digital data to a signal tap 58. Digital data is output bytap 58 on primary output line 60 to a decoder 62. Tap 58 also providesan identical stream of digital data via the secondary output line 64 tocomparator 66 as will be discussed shortly. The decoder 62 providesmultiple processing functions to provide, among other things, bit errorcorrection, and also determines a bit error rate (BER). Decoder 62provides a BER signal on output line 72 indicative of the determinedBER. Decoder 62 may be a forward error correction (FEC) decoder as well.The output of the decoder 62 provides the processed digital data fromthe receiver 40 to output line 76. Ideally, the output digital dataprovided on output line 76 is identical to the input electrical digitaldata on input line 26.

[0036] Referring now to the secondary auxiliary decision circuit 54, theoutput of this circuit 54 is provided on output line 80 to comparator66. Comparator 66 compares the digital data output of the primarydecision circuit 52 with the output of the auxiliary decision circuit 54to determine differences therebetween. The output of the comparator 66is provided on output line 82 to an error counter 84. The auxiliarycircuitry comprising secondary decision circuit 54, comparator 66, anerror counter 84, and a controller 85 are used to ultimately determine aQ factor of the optical carrier and provides this Q factor as anindicator on output line 86 by observing the error count.

[0037] Robustness to timewise disturbances is gauged by directing theauxiliary decision circuit 54 to sample at various time offsets relativeto the optimum set point of the primary decision circuit 52. Thesecondary decision circuit 54 has adjustment circuitry which facilitatesthe dithering of the decision point to different levels to probe theextents of the operating window. The comparator 66 determines thevariance of the output from the secondary decision circuit 54 in view ofthe output from the primary decision circuit 52. Controller 85 dithersthe decision point of the secondary decision circuit 54 and has noeffect on the output digital data on output line 76, but allows thedecision level to be adjusted to determine if this improves the Q factorof the optical carrier by observing the error count from error counter84, and facilitates adjustments of the optimum setting of the primarydecision circuit 52 where necessary. In essence, the auxiliary circuitryin receiver 40 allows the data signal to be analyzed without affectingthe primary receiver circuitry to intelligently determine ifimprovements can be made to the primary circuitry.

[0038] According to the present invention, controller 22 includessoftware or equivalent hardware to receive and process various inputsfrom the various portions of the optical circuit 20 to provide faultmanagement of the optical communications system 20. The controller 22receives and analyzes the various inputs, and processes them todistinguish between, among other things, fiber failure, PMD-relateddegradations, and failure of monitoring equipment itself The controller22 provides outputs on line 92 that may be used to alter protectswitching logic and to alert maintenance personnel as to the probablecause of degraded path indications. In the preferred embodiment, fourinputs are supplied for each optical channel to the controller 22including the bit error rate (BER) observed at the receiver, the Qmeasurement obtained at the receiver, alarms or notifications from thePMDCs along the optical path, and the optical signal-to-noise ratio asmeasured by the selective optical power meter tapped onto the path nearthe receiver. However, limitations to these indicators is not intendedand other system indicators can be generated and analyzed as well andare encompassed by the present invention. Controller 22 maintains a timestamped record of recent measurements and notifications received via thesupplied inputs.

[0039] Referring now to FIG. 4, there is shown a flow diagram of theprocessing algorithm of controller 22 according to the preferredembodiment of the present invention being generally shown at 100. Thismethod is preferably implemented in software, but could be implementedin hardware if desired.

[0040] The method starts at step 102 whereby the optical system isinitialized. Next, at step 104, the controller 22 determines ifdegradation of the system is observed across multiple optical channelsby observing indicators from the system devices associated with theseother channels. Collectively, these indicators are received on inputline 90 as shown in FIG. 3. If degradation of the system is observedacross the multiple optical channels at step 104, at step 106 thecontroller 22 will declare a possible fiber failure due to thecorrelation that several optical channels are degraded and degradationis not limited to one channel.

[0041] At step 104, if degradation is not observed across multiplechannels, the algorithm proceeds to step 108 whereby the currentobserved BER provided by decoder 62 via output line 72 is time stampedand recorded at controller 22. Next, at step 110 the controller 22calculates the predicted BER based upon the most recent recorded Qmeasurement provided by error counter 84 via output line 86, ascalculated according to the algorithm 130 of FIG. 5 which will bedescribed shortly. The BER can be predicted based upon the most recentrecorded Q measurement according to various known algorithms andmathematical relationships. Some known ways of calculating a BER fromthe Q measurement are described in the two referenced articlesidentified in the section Background of the Invention entitled “Q-Factormeasurements for High Speed Optical Transmissions”, and “MarginMeasurement in Optical Amplifier Systems”, the teachings of which areincorporated herein by reference.

[0042] Next, the algorithm proceeds to step 112 where it is determinedwhether or not the observed BER recorded in step 108 is significantlybetter than the predicted BER calculated in step 110. If the observedBER is determined to be significantly better than the predicted BER, thealgorithm proceeds to step 114 and reports that the most recent recordedQ measurement from step 110 is suspect and false. This report is basedupon the fact that a healthy observed BER is always the reliableindicator of the true operating characteristic of the network 20. If ahealthy BER is being reported by decoder 62, any Q measurement to thecontrary must necessarily be in error.

[0043] If at step 112 the observed BER is not determined to besignificantly better than the predicted BER, the algorithm proceeds tostep 116 to determine if the observed BER is significantly worse thanthe predicted BER. If the answer is no, the algorithm proceeds back tostep 104 since the observed BER is generally close to the predicted BER,and thus, the most recent recorded Q measurement is determined to bevalid.

[0044] If, however, at step 116 the observed BER is determined to besignificantly worse than the predicted BER, the algorithm proceeds tostep 118 to determined whether or not the observed BER has significantlydegraded more recently than the latest recorded Q measurement accordingto the algorithm 130 in FIG. 5, which will be described shortly. If atstep 118 the answer is no, the algorithm proceeds to step 120 andreports that the latest Q measurement recorded in algorithm 130 of FIG.5 is false, or that the receiver 40 is bad. This can be determinedbecause there has been sufficient time for the Q measurement to observethe degrading BER. If at step 118, however, it is determined that theobserved BER has degraded more recently than the latest recorded Qmeasurement provided in algorithm 130, the algorithm has no basis forinvalidating the recorded Q measurement and proceeds back to step 104since a recent degraded BER would account for why the observed BER ismuch worse than the predicted BER based on the most recent recorded Qmeasurement.

[0045] In summary, algorithm 100 determines whether or not recorded Qmeasurement is valid or false by determining whether or not thepredicted BER from the recorded Q measurement is in line with therecorded observed BER. Algorithm 100 can also determine whether or notthe receiver 40 is bad.

[0046] Referring now to FIG. 5, there is shown generally at 130 a flowdiagram of the processing algorithm of controller 22 to record acompleted Q measurement in a recent history list for use in algorithm100, at step 110.

[0047] At step 132, a new Q measurement is determined by error counter84 and completed. This Q measurement typically takes a few minutes tocomplete as the auxiliary receiver is adjusted to different decisionpoints. Next, at step 134, it is determined whether or not the new Qmeasurement of step 132 has degraded significantly over the previous Qmeasurement as provided by error counter 84. If the answer is no, thealgorithm proceeds to step 136 to determine if the PMDC 32 is an alarmstate. If the answer is yes, the algorithm proceeds to step 138 toreport that the PMD alarm indication is false. It is known that the PMDindication is false since it is not normal for the PMDC 32 to be inalarm state when the Q measurement has not degraded as determined instep 134. If at step 136 the PMDC is not in the alarm state, thealgorithm proceeds to step 140.

[0048] At step 140, the controller 22 determines if the optical signalto noise ratio (OSNR) from SNR meter 36 has recently degraded. If theanswer is yes, the algorithm proceeds to step 142 and reports a falseOSNR indication. It is reported at step 142 that the OSNR indication isfalse because if the OSNR has degraded, it would have be determined atstep 134 that the Q measurement would have been degraded.

[0049] If, however, at step 140 the OSNR is not determined to havedegraded, the algorithm proceeds to step 150 and the new Q measurementof step 132 is determined to be valid and is recorded in the most recenthistory list at controller 22 for use at step 110 in flow diagram 100.

[0050] Referring now back to step 134, if it is determined that the mostrecent Q measurement of step 132 has appreciably degraded since the lastQ measurement of the previous iteration of algorithm 130, the algorithmproceeds to step 152.

[0051] If at step 152 it is determined that either the PMDC 32 or theSNR meter 36 are presently in alarm state by observing the output online 34 or line 38 from the respective devices, the algorithm proceedsto step 154 and reports that the Q measurement degradation determined instep 134 is attributable to the particular PMDC 32 or the SNR meter 36in an alarm state. The correlation that the Q measurement is determinedto have significantly degraded in step 134 in combination with an alarmfrom either the PMDC 32 or the SNR meter 36 is used to isolate whetheror not the Q measurement degradation is attributable to the PMDC 32 orthe SNR meter 36. The respective alarm is indicative of which systemdevice is indicating a problem.

[0052] At step 152, typical alarms of the PMDC 32 include, but are notlimited to:

[0053] I. the PMD compensator is approaching the limit of itscompensating ability;

[0054] II. the optical signal has exceeded the compensating range of thePMD compensator;

[0055] III. the range of change of the PMD exceeds at predeterminedcharacteristic value or exceeds a tracking speed of the compensator;

[0056] IV. an element in the PMD compensator has failed.

[0057] It is noted that degradation in Q factor is usually caused by apoor signal-to-noise ratio or timing dispersion, or a combinationthereof If an abrupt degradation is observed during the course of makingQ measurements, the notifications from the PMDC 32 along the line arereviewed by controller 22 to see if a polarization-related anomalyoccurred during that time. This information is used by controller 22 todisregard selected readings taken during the Q measurement, or toinvalidate a Q measurement entirely.

[0058] Cross-checking by controller 22 among the Q measurement, SNR, andPMD notifications also allows for monitoring of the measurementequipment itself If it is determined that the Q measurement factordegrades and the SONET error rate escalates comparably, it is expectedthat the PMD notifications and SNR readings would account for thedegradation. If neither PMD or SNR readings reflect any degradation,then one of these monitoring devices is determined by controller 22 tobe malfunctioning, or it is determined that an impairment is occurringbetween the tap for monitoring equipment and the decision circuit of thereceiver, perhaps at the receiver front end.

[0059] In a different scenario, if a PMD event is registered that wouldbe severe and prolonged enough to theoretically prevent signalreception, yet the SONET BER and Q measurements are not determined bycontroller 22 to degrade appreciably, the PMDC itself may bemalfunctioning. This cross-checking according to the present inventioncan prevent unnecessary protection switching of optical channels thatmight result if only a single indication were relied upon which candisrupt data traffic.

[0060] The method of the present invention first integrates andprocesses several input indicators to distinguish between, upon otherthings, fiber failure, PMD-related degradations or failure of themonitoring equipment itself The outputs provided by controller 22 of thepresent invention can be used to alter protect switching logic and toalert network maintenance personal as to the probably cause of degradedpath indications. The present invention intelligently assimilates andanalyzes indicators and notifications from various network equipment toprovide better fault management in the optical communication system.

[0061] According to the present invention, if controller 22 furtherdetermines according to the algorithm 170 shown in FIG. 6 that the limitcondition declared by the PMDC 32 is the main problem affecting thelink, controller 22 instructs SOP compensator 31 via control line 34 toalter rotation by some arbitrary amount, for example, 45 degrees or 90degrees. This action will most likely result in lowering the end-to-endPMD to acceptable levels. The rare condition of high PMD occurs when thefiber shifts properties, and exhibits a worst case dispersion inconjunction with the settings of the SOP compensator 31. Therefore,changing any of the polarization of SOP compensators 31 in eitherdirection will probably result in reduced PMD.

[0062] If, however, controller 22 determines via algorithm 170 that thelimit condition declared by the PMDC 32 is not the main problemaffecting the link 20, or is simply a consequence of a separate problem,the controller 22 refrains from directing the SOP compensator 31 toattempt to fix the problem. Thus, the present invention provides asafeguard against false triggering. The present invention provides atechnique to gate whether or not the SOP compensator 31 should abruptlyshift state. The present invention maintains the integrity of thetraffic bearing signal by avoiding making unnecessary correctiveactions, such as switching channels or altering the rotation of a SOPcompensator 31, which unnecessarily temporally disrupt the trafficbearing signal, and which may not actually correct a PMD problem.

[0063] Referring to FIG. 6, algorithm 170 of controller 22 starts at 172and proceeds to step 174 to determine if the observed BER from decoder62 has degraded past an acceptable predetermined threshold i.e. 10 ⁻¹⁰.If the answer is no, the algorithm simply proceeds back to step 174 anditerates until it is determined at step 174 that the observed BER hasdegraded past the acceptable threshold. This is because as long as theobserved BER is determined to be healthy and above a predeterminedacceptable threshold, there is no need to perturb the settings of anyPMD compensators 31, regardless whether or not the PMDC alarms are beinggenerated. As long as the observed BER is acceptable, it is known thatthe signal through the optical network link 20 is acceptable.

[0064] At step 174, once controller 22 determines that the observed BERhas degraded past the acceptable threshold, the algorithm proceeds tostep 176 to determine if the SNR meter 36 is bad as determined byalgorithm 130 as shown in FIG. 5. If the answer is yes, the algorithmproceeds to step 178 and controller 22 determines if the SNR meter 36has given recent false positives. If the answer is no, controller 22deems that the signal to noise ratio in link 20 is bad and that thedegraded BER determined in step 174 is not PMD related. The controller22 then lets the FWM controller handle the SNR problem at step 180 andthe algorithm proceeds to step 182 to finish the routine.

[0065] If at step 176 is not determined that the SNR meter 36 is bad, orif at step 178 it is determined that the SNR meter 36 has given recentfalse positives, the algorithm proceeds to step 184 to determine if thePMDC 32 is in the alarm state. If the answer is no, the algorithmproceeds to step 186 and controller 22 determines if the PMDC 32 hasgiven recent false negative readings. If the answer is no, it isdetermined that the PMDC 32 is properly operating and there is not a PMDproblem. Therefore, the controller 22 restrains from changing anysettings of the SOP compensator 31.

[0066] Referring back to step 184, if it is determined that the PMDC 32is generating valid alarms, such as those four alarms previouslymentioned, the algorithm proceeds to step 188 whereby it is determinedthat there is a PMD problem. Responsively, the controller 22 instructsthe SOP compensator 31 to change preferences to alter rotation by somearbitrary amount, for example 45 degrees or 90 degrees. This action thenwill most likely result in lowering the end-to-end PMD to acceptablelevels. Again, since the rare condition of high PMD occurs when thefiber shifts properties and exhibits a worst case dispersion inconjunction with the settings of the SOP compensators 31, changing anyof the SOP compensators 31 in either direction will result in lower PMDin link 20.

[0067] Referring to step 186, controller 22 determines that the PMDC isnot in the alarm state at step 184, but in step 186 the PMDC 32 isdetermined by controller 22 to have given false negative readingslately, the algorithm again proceeds to step 188 to cause the SOPcompensators 31 to change preferences. This changing of preferenceslowers the end-to-end PMD to acceptable levels. It is noted at step 188that algorithm 170 may restrain from changing preferences of the SOPcompensator 31 if a change to the SOP compensator was recently madesince changing the rotation angle may be futile.

[0068] In summary, the present invention provides an apparatus andmethod whereby the SOP compensator 31 along the link 20 are directed tochange states whenever it is intelligently determined that a PMDC 32surpasses a preset compensation limit, and when it is also determinedthat a PMDC 32 is the main problem affecting the link, and not simply aconsequence of a separate problem. The PMD controllers 32 are initiallyset such that the likelihood of reaching a compensation limit isacceptably small. Upon the rare occasion that the compensation limit isexceeded, one or more SOP compensator 31 is given a command bycontroller 22 to alter rotation by some arbitrary amount, for example,45 degrees or 90 degrees. This action most likely results in loweringthe end-to-end PMD to acceptable levels, safeguarding against falsetriggering and reducing unnecessary interruptions to traffic bearingsignals.

[0069] Though the invention has been described with respect to aspecific preferred embodiment, many variations and modifications willbecome apparent to those skilled in the art upon reading the presentapplication. It is therefore the intention that the appended claims beinterpreted as broadly as possible in view of the prior art to includeall such variations and modifications.

We Claim:
 1. A method of improving polarization mode dispersion (PMD) inan optical signal transmitted by an optical communications system, theoptical communications system having a State of Polarization (SOP)controller, a PMD compensator (PMDC), and at least one optical linkelement, comprising the steps of: obtaining a PMD notification from thePMDC indicative of PMD in the optical communications system; obtaining afirst indicator from a first optical link element indicative of anattribute of the optical signal in the optical communications system;and improving the PMD of the optical signal by selectively shifting anangle of polarization of the optical signal by the SOP controller as afunction of both said PMD notification and said first indicator.
 2. Themethod as specified in claim 1, wherein said first indicator isindicative of a bit-error-rate (BER) of the optical signal.
 3. Themethod as specified in claim 1, wherein said first indicator isindicative of the value of a Q-factor measurement upon the opticalsignal.
 4. The method as specified in claim 2, further comprising thesteps of: generating a second indicator by the first system deviceindicative of a Q-factor of the optical signal; and wherein the step ofimproving the PMD of the optical signal comprises selectively shiftingthe PMD as a function of said second indicator and said first indicator.5. The method as specified in claim 1, wherein said step of obtaining afirst indicator from a first system device comprises obtaining a signalto noise ratio of the optical signal from a signal to detector noiseratio (SNR).
 6. The method as specified in claim 2, further comprisingthe steps of: generating a signal for noise ratio (SNR) of the opticalsignal by a second system device as a second indicator; and wherein thestep of improving the PMD of the optical signal comprises selectivelyshifting the PMD as a function of said second indicator and said firstindicator.
 7. The method as specified in claim 3, further comprising thestep of: generating a signal for noise ratio (SNR) of the optical signalby a second system device as a second indicator; and wherein the step ofimproving the PMD of the optical signal comprises selectively shiftingthe PMD as a function of said second indicator and said first indicator.8. The method as specified in claim 4, further comprising the steps of:generating a signal to noise ratio (SNR) by an SNR detector as a thirdindicator; and wherein the step of improving the PMD of the opticalsignal comprises selectively shifting the PMD as a function of saidthird indicator, said second indicator and said first indicator.
 9. Themethod as specified in claim 1, wherein said PMD notification comprisesone or more of a group consisting of: i) the PMDC is approaching thelimit of its compensating ability; ii) the optical signal has exceededthe compensating range of the PMDC; iii) the rate of change of the PMDexceeds a predetermined characteristic value or exceeds a tracking speedof the PMDC; and iv) an element in the PMDC has failed.
 10. The methodas specified in claim 2, wherein said PMD notification comprises one ormore of a group consisting of: i) the PMDC is approaching the limit ofits compensating ability; ii) the optical signal has exceeded thecompensating range of the PMDC; iii) the rate of change of the PMDexceeds a predetermined characteristic value or exceeds a tracking speedof the PMDC; and iv) an element in the PMDC has failed.
 11. The methodas specified in claim 3, wherein said PMD notification comprises one ormore of a group consisting of: i) the PMDC is approaching the limit ofits compensating ability; ii) the optical signal has exceeded thecompensating range of the PMD; iii) the rate of change of the PMDexceeds a predetermined characteristic value or exceeds a tracking speedof the PMDC; and iv) an element in the PMDC has failed.
 12. The methodas specified in claim 4, wherein said PMD notification comprises one ormore of a group consisting of: i) the PMDC is approaching the limit ofits compensating ability; ii) the optical signal has exceeded thecompensating range of the PMDC; iii) the rate of change of the PMDexceeds a predetermined characteristic value or exceeds a tracking speedof the PMDC; and iv) an element in the PMDC has failed.
 13. An opticalcommunications system, comprising: an optical fiber; an opticaltransmitter coupled to said optical fiber which generates an opticalsignal; a state of polarization (SOP) compensator coupled to saidoptical fiber; a polarization mode dispersion controller (PMDC) coupledto said optical fiber generating a first signal indicative of PMD insaid optical fiber; a system measurement device coupled to said opticalfiber generating a second signal indicative of a condition of theoptical signal in said optical fiber; and a controller coupled to saidPMDC, said PMDC and said system measurement device determining a faultwithin said optical fiber as a function of said first signal and saidsecond signal, wherein said controller selectively shifts an angle ofsaid SOP compensator when said controller determines said PMDC is thefault in the optical communications system.
 14. The opticalcommunications system as specified in claim 13, wherein said systemmeasurement-device comprises an optical receiver generating a bit errorrate (BER) of said optical signal as said second signal.
 15. The opticalcommunications system as specified in claim 13, wherein said systemmeasurement device comprises an optical receiver generating a Q-factorof said optical signal as said second signal.
 16. The opticalcommunications system as specified in claim 14, wherein said opticalreceiver further generates a Q-factor of said optical signal as a thirdsignal; and wherein said controller determines a fault within saidoptical communications system as a function of said first signal, saidsecond signal, and said third signal.
 17. The optical communicationssystem as specified in claim 17, further comprising: a secondmeasurement device comprising a signal-to-noise ratio (SNR) detectorgenerating a SNR of said optical signal as a third signal; and whereinsaid controller determines said fault within said optical communicationssystem as a function of said first signal, said second signal, and saidthird signal.
 18. The optical communications system as specified inclaim 14, further comprising: a second measurement device comprising asignal-to-noise ratio (SNR) detector generating a SNR of said opticalsignal as a third signal; and wherein said controller determines saidfault within said optical communications system as a function of saidfirst signal, said second signal, and said third signal.
 19. The opticalcommunications system as specified in claim 15, further comprising: asecond measurement device comprising a signal-to-noise ratio (SNR)detector generating a SNR of said optical signal as a second signal; andwherein said controller determines said fault within said opticalcommunications system as a function of said first signal, said secondsignal, and said third signal.
 20. The optical communications system asspecified in claim 16, further comprising: a second measurement devicecomprising a signal-to-noise ratio (SNR) detector generating a SNR ofsaid optical signal as a third signal; and wherein said controllerdetermines said fault within said optical communications system as afunction of said first signal, said second signal, and said thirdsignal.
 21. The optical communications system as specified in claim 13,wherein said PMDC first signal is indicative of a condition selectedfrom the group consisting of: i) the PMDC is approaching the limit ofits compensating ability; ii) the optical signal has exceeded thecompensating range of the PMDC; iii) the rate of change of the PMDCexceeds a predetermined characteristic value or exceeds a tracking speedof the PMDC; and iv) an element in the PMDC has failed.